The si-units tag has no wiki summary.
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The units of gain and number of atoms in population inversion in a laser
I am following my university course notes on amplification in laser media, and have come across expressions for the gain of a medium, but the notes are not exactly rigorous... The expression given for ...
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2answers
124 views
Why metric system uses kilogram as a basic SI unit?
SI system uses all (that I know) measurement basic units as 1 (single) instance: meter, second, ampere, etc, except the KILOgram. It already defined with 1000 multiplier (kilo).
It prevents from ...
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1answer
49 views
Zero uncertainty constant and a unit change
So, we know the speed of light with zero uncertainty. We also know that values of $\epsilon_0$ (electric constant) and $\mu_0$ (magnetic constant) are known with zero uncertainty.
My questions are ...
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0answers
19 views
How to convert lb/MMBTU to concentration?
This might be a stupid question but...
I am looking over this document for allowable emissions for PM particles in biomass furnaces.
They are given in lbs/MBTU, pounds per million British Thermal ...
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2answers
74 views
What Physical Quantity has SI Unit $kg \cdot m$,? [duplicate]
What is the physical quantity that has SI (System International) unit of $kg \cdot m$ (kilogram meters)?
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1answer
41 views
How do I obtain a sense of scale when thinking about webers?
This Wikipedia article provides a wonderful way to fathom the scale of the tesla. I cannot seem to find a similar set of examples about magnetic flux. I know that the milli-, micro- and nanoweber are ...
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2answers
45 views
Asymmetric uncertainties
Inspired by these two question on tex.SX
Asymmetrical tolerancing
Asymmetric uncertainties with siunitx package
I'd like to ask for a nice explanation for these kind of uncertainties, like ...
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1answer
53 views
A sphere, a simple object?
In this video, the woman says that a sphere is a pretty simple object. What intrigues me is the use of a sphere for such a calculation. First of all, the sphere wouldn't be perfect as a perfect sphere ...
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4answers
577 views
Why there is no “Edison” unit in physics? [closed]
In the popular culture the XIX-XX century competition between Thomas Edison and Nikola Tesla is well-known. The example could be the Prestige movie, where there are some "Edison's agents" who sabotage ...
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8answers
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Is there a symbol for “unitless”?
I'm making a table where columns are labelled with the property and the units it's measured in:
Length (m) |||| Force (N) |||| Safety Factor (unitless) ||| etc...
I'd like not to write "unitless" ...
3
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1answer
84 views
How would I explain a farad being a second per ohm?
The farad has many alternative representations in SI. I can comprehend some of them. For example, a capacitance of 1 farad means that it will take a single coulomb to create a single volt between the ...
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1answer
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Does the Kelvin have a rigorous definition?
From Wikipedia:
The kelvin is defined as the fraction 1⁄273.16 of the thermodynamic temperature of the triple point of water.
That presupposes that we can take a fraction of temperature.
Now, ...
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2answers
80 views
Are the SI units used in astrophysics?
Just a curious question, do astrophysicist use the SI units, for example in this equation,
$r = 5pc$, will this be converted to meters?
And what does this $\nu$ stand for?
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1answer
114 views
Spectral radiance unit conversion [closed]
I have spectral radiance data in SRUs (spectral radiance units), as a function of wavelength:
$$a = \mu W cm^{-2} sr^{-1} nm^{-1} $$
However, I am working with software which requires my data in the ...
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3answers
209 views
What physical quantity has SI unit $\mathrm{kg}/\mathrm{m}$?
What physical quantity has SI unit $\mathrm{kg}/\mathrm{m}$?
For example, the physical quantity with SI unit $\mathrm{kg}\cdot\mathrm{m}/\mathrm{s}^2$ is force $F$ and the physical quantity with SI ...
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2answers
182 views
Intuitive meaning of Newton (units)
Is there any intuitive reasoning behind why the units of a "Newton" is $kg \frac{m}{s^{2}}$ and how it represents force? I always wanted to understand why objects of different mass fall at the same ...
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1answer
169 views
CGS Units to SI Units
This may be a stupid question but I am having trouble getting the same result in CGS units as I would if I used SI units for a unitless calculation.
I have to calculate $E_c=\frac{m_e c\, ...
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1answer
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$E^2 = (mc^2)^2 + (pc)^2$: What units are used to measure $E$, $m$, $c$ and $p$?
\begin{equation}
E^2 = (mc^2)^2 + (pc)^2
\end{equation}
If I am using this equation to figure out the energy of something, what units would I use? Would it be the metric system? I.e. kilograms ...
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2answers
98 views
SI units with more than one prefix in fractions
Is it (in the view of SI) correct to note units with more then one prefix? I discuss this since several months with friends, but we could not find a proper source for our statements yet.
Examples for ...
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1answer
83 views
Why the kilogram is not defined? [duplicate]
Possible Duplicate:
Why do we still not have an exact definition for a kilogram?
I was thinking about SI units. I found the following definition for the base units:
Meter: distance ...
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1answer
230 views
The origin of femto, atto and zepto SI prefixes
Do you know why the SI prefixes: femto, atto, zepto have been accepted by Scientific Community, if this triad of metric units, is neither greek nor latin?
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1answer
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There are plans to develop a better definition of a “second”. How does the current definition fall short?
The current definition of a second is stated here and I found a presentation on the BIPM site which discusses plans to change to a "better" definition of a second. You can find the presentation here. ...
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3answers
163 views
Can you use PCR to make a standard kg?
While reading this question: Why do we still not have an exact definition for a kilogram? , I had a crazy thought.
Using PCR, you make a known number of copies of a DNA strand where the length and ...
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3answers
326 views
Why do we still not have an exact definition for a kilogram?
I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It seems this would be fairly important.
Edit
Today I ...
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1answer
235 views
Conversion from natural units to SI
I got questions about converting units from natural system of units to SI.
To be exact, I'm solving the problem in Heisenber interpretation of quantum mechanics, and I'm using Heisenberg equation of ...
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3answers
177 views
What is a “measure equation” as mentioned by this TeX Users Group guide?
In this TeX Users Group (TUG) document, Typesetting mathematics for science and technology according to ISO 31/XI by Claudio Beccari, the author makes various typesetting recommendations including:
...
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1answer
196 views
Do the trigonometric functions preserve units?
I saw an exercise where you had to calculate the units of $C_i, i=1,2$ from an equation like this:
$v^2=2\cdot C_1x$ and
$x=C_1\cdot \cos(C_2\cdot t)$
where
$x$ means meters,
$t$ means seconds ...
3
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1answer
89 views
Is there any literature that compares the candle-power to the candela?
I am unable to find anything in the literature to back up the claim that appears all over the internet that:
...
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2answers
541 views
Relating milliampere-hours to watt-hours for batteries
I've seen many batteries that are measured in milliampere hours (mAh), while others are measured in watt hours (wh). How can I convert them between each other so that I can actually compare them? It's ...
4
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1answer
156 views
Avogadro's number
Could I get an explanation of Avogadro's number and how it relates to determining the mass of a substance? My chemistry textbook only serves to confuse me and the Wikipedia article is aimed towards a ...
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2answers
2k views
Unit for Impact Force
Forgive me if this is a newb question but I am not a trained scientist, much less a physicist. I'm just curious and would like to know if there's a unit to measure impact force.
I know the newton is ...
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5answers
876 views
Is anything actually 1 meter long (or 1kg of weight)?
I believe that no real objects are actually (exactly) 1 meter long, since for something to be 1.00000000... meters long, we would have to have the ability to measure with infinite precision. ...
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9answers
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What is the logarithm of a kilometer? Is it a dimensionless number?
In log-plots a quantity is plotted on a logarithmic scale. This got me thinking about what the logarithm of a unit actually is.
Suppose I have something with length $L = 1 km$.
$\lg L = \lg km$
It ...
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3answers
146 views
units of measure
This question is rather historical one.
kilometres can be defined in metres, metres in centimetres, centimetres in millimetres.
There must be some elementary unit (like millimetre or smth.) which ...
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1answer
870 views
what is an inverse femto barn?
I came across the use of the unit barn and inverse barn while reading about the operation of LHC. What is an inverse femtobarn ? What does it tell about the experiment being described ?
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1answer
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How large is the information collected from an inverse femtobarn of collisions?
I ran into this while looking at measures of humongous amounts of data. How does the information (data) collected in an inverse femtobarn exposure compare to a gigabyte of data ?
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5answers
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Whats the range of dimension exponents in the SI Units system?
I am working on a software library for Units of Measure. To represent dimensions, I need to know the required range of exponents for each of the seven base units (precisely, I need to know the ...
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2answers
490 views
Fundamental units
Is it right that all units in physics can be defined in terms of only mass, length and time?
Why is it so? Is there some principle that explains it or is it just observational fact?
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2answers
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What is “International Service of Weights and Measures”?
In the Resolution 2 of the 3rd meeting of the CGPM, defining the kilogram, “the International Service of Weights and Measures” is mentioned (the French original text reads “le Service international ...
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3answers
272 views
Help getting the distance an object has moved
What is a distance an object went through at the speed of 14hm/min for 6 hours?
I am having problems with conversions.
First, I tried passing 14hm/min to m/s. I did it alright. But now I am unsure. ...
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2answers
709 views
Why does $E=mc^2$ give results in Joules?
Why does $E=mc^2$ give results in Joules ? Why didn't Einstein need to create a separate unit and how come using two standard units (speed of light in km.s and mass in grams) give another result in ...
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3answers
625 views
How to write units?
In a scientific article, should I write "3m", "3 m", "3 meters", or "3 [meters]" ?
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1answer
306 views
what does 'same energy as a ton of TNT' mean?
Very often the energy released in large-scale events (e.g., earthquakes, meteor impacts, etc) is described in terms of some number of 'tons of TNT'. This may be all well and good for those with an ...
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2answers
1k views
gauss vs mW/cm^2: same thing?
A friend of mine is concerned about electromagnetic field negatively affecting their health and got a "DMF meter" to measure the field strength in their house in units of milliGauss. They are trying ...
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1answer
408 views
What is the SI Derived Unit of Brix? [closed]
I would like to know in which Derived International System Unit a Brix value could be converted to?
Thanks!
UPDATE
Is Brix the SI units to measure the sugar content of an aqueous solution?
It ...
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3answers
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coulomb force in SI and cgs
Coulomb force in SI is
$ F = \frac{Q1*Q2}{4\pi\varepsilon R^{2}} $
while in CGS
$ F = \frac{Q1*Q2}{R^{2}} $
why is it? I mean doesn't it any make difference in dimension? since $ \varepsilon $ ...
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1answer
405 views
“Natural units” of mass
Gravitational attraction is given by $\frac{GMm}{r^2}$ while attraction due to electric charge is given by $\frac{q_1 q_2}{r^2}$. Why does gravity need a constant while electric charge doesn't? ...
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3answers
353 views
Light emission spectrum units
Do someone knows the units of the spectra provided here ? It seems obvious enough that it's said nowhere, but even Wikipedia and other sites are quite blurry on this point.
So, is it power ($W$), ...
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6answers
852 views
What's keeping us from simply redefining Avogadro's Number / the Mole as a definite integer?
This might be a question to ask in a Chemistry site, but because there is a lot of talk about redefining many units of measurements in terms of Avogadro's Number / the Mole, I was wondering why we ...
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5answers
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Does this statement make any sense?
I am asking this question completely out of curiosity. The other day, my roommate, by mistake, used 'Light year' as a unit of time instead of distance. When I corrected him (pedantic, much), he said ...
